Variable Gain Amplifiers (VGAs) are well known in the art. VGAs are generally used when the magnitude of a signal, such as an electrical current or voltage, is required to be increased or decreased. The range of gain control of a VGA is essentially defined as the difference between maximum and minimum gain. For example, if the maximum gain is +10 dB (magnification) and the minimum gain is -40 dB (attenuation), the range of gain control is 50 dB. It is understood that the maximum gain in a VGA may actually be an attenuation of the input signal.
In today's hand-held cellular digital radios, there is a need to control the transmitted power over a very wide control range. For IS-95 radio systems, the required control range can exceed 100 dB. Typically, two or more VGAs are cascaded in a transmitter line-up in order to achieve higher control ranges.
The use of multiple, or multistage as they are commonly known, VGAs has various drawbacks, such as excessive current consumption and poor noise performance. Since each VGA generally requires its own DC bias current, cascading two, three, or more VGAs will require an increase in supply current. This becomes a problem in small size, high talk time cellular phones since a large battery will be required to supply the increased supply current. Further, since each VGA stage generates noise, increasing the number of stages effectively increases the amount of noise, resulting in degraded performance.
The present invention is directed toward overcoming one or more of the above-mentioned problems.